Audi will launch its North American diesel offensive with the Q7 3.0 TDI arriving next January and the A4 3.0 TDI that will follow within a year (VW's Touareg twin will get the new V-6 as well). Naturally, meeting the world's most stringent diesel emissions regs (those of the five California-emissions states), the engine is a technological tour de force. In order to burn every molecule of diesel fuel as completely, cleanly, and efficiently as possible, the engine uses advanced piezo-electric injectors with eight tiny spray holes each. By injecting the fuel at 29,000 psi over the course of five or more tiny squirts per combustion cycle, cylinder pressures rise gently, preventing the marbles-in-a-blender clatter of old diesels. The engine computer monitors combustion by mapping this rise in cylinder pressures during every cycle in order to quantify the fuel quality, which varies more widely in the U.S. than in Europe. Our cetane numbers range from 39 to 53, in contrast to Europe's, which are regulated to a minimum of 51 (the higher the number, the sooner combustion is initiated, so the fuel burns longer and more completely for improved performance and emissions).
The 3.0 TDI's single variable-nozzle turbocharger is optimized for greater torque at a slight cost to horsepower relative to the European-spec TDI, and it feeds twin intercoolers. Except, that is, when hotter intake is required for emissions purposes, at which point the intercoolers are bypassed. Once the pretty clean exhaust leaves the turbo it is further scrubbed by an oxidation catalyst, a particle filter, and a NOx-reduction catalyst. It also flows past an oxygen sensor, four temperature sensors, and two new Siemens NOx sensors. The latter provide closed-loop control of the urea injection system that completes the job of reducing NOx emissions by 90 percent. This 32.5-percent urea/water solution is carried in two tanks with a total capacity of 6.1 gallons (one tank is heated, as the solution freezes at 12 degrees F). That's enough to last a minimum of 10,000 miles. It's refilled via a special inlet next to the diesel filler during regularly scheduled maintenance stops, and it should sell for about the price of windshield-washer fluid.
